Taking cost and every other consideration aside, if a coal gasification project has emissions comparable to a natural gas fired plant including carbon, can it be considered clean?

Sweden and the other Scandinavian countries rely very heavily on a wide range of taxes and feebates, and they work very well.

When Sweden, for example, divided sulfur-containing vehicle fuels into three glasses, sort of like the Three Bears' porridge, the dirtiest fuel was taxed and the money given those selling the cleanest fuel.  Within  six months Sweden had virtually cornered the world market on low sulfur fuel.

The same policy was applied to emissions of oxides of nitrogen for sources too small to be effectively equipped with NOx destruction equipment.  Relatively high emitters paid into a fund from which payments were made to relatively low emitters.  NOx emissions fell 40 percent, as employees were paid bonuses to make burner and other adjustments to reduce emissions.

The key to feebate successes was that they are revenue neutral: every kroner that comes in also goes out, with none sticking to the walls.
If the United States were to adopt such a program for, say, generating electricity, the price differential between coal and renewables would disappear, together with the need to think up ways of providing incentives.  The playing field would then be truly level.

The same could be done with vehicles, causing the prices of hybrids, plug-in hybrids and high fuel economy cars, trucks and buses to drop, while those of Hummers and other SUVs rose.  The information required to make such a system work is already collected and the system for operating exists too, because vehicles must be registered.

Sweden also repealed one half of its general tax and replaced the lost income with a tax on carbon dioxide.  Result: renewable energy took off like a rocket.  These success stories and more can be found in my book, Saving Ourselves: How We Can and Why We're Not: The Roles of Corporate    America and the Republican Party in Perpetuating Global Warming at saving-ourselves.com

The cleanest fossil fuel power plant in the world to the best of my knowledge is Vartan in Stockholm, which is roughly 92 percent efficient.  While its emissions are thus extremely low compared to existing coal-fired plants (about 34 % efficient) and new ones (about 50 percent) and even combined heat and power systems (typically about 75 percent), there is still pollution coming out the stack and it is still killing people and warming the planet.

This is a major reason that Oregon's power plant siting law requires a new plant to be as clean as the best in the world, then make a contributions to a CO2 reduction fund to purchase another 15 percent cut.

The only truly clean plants are those that rely on sunlight, wind, waves, etc.  Allowing emissions any greater than zero is to subsidize a fossil plant--a subsidy that consists of not only money, but a sacrifice of the health of those who breathe the air and the future of those living in a hotter planet.

The message coming from our soon-to-be government: No climate emergency to see here. Move along people. Status quo politics at its' finest. These ass-hats are going to kill us and the planet. OK, well, they won't kill the planet. As long as the plates still move around the globe, thus providing energy to the various weather systems and climates, the planet will continue. We, however, if nothing is done, will cease to exist.

The mellotron is your friend.

[new] CCS

Taking cost and every other consideration aside, if a coal gasification project has emissions comparable to a natural gas fired plant including carbon, can it be considered clean?
Certainly. Well, AS clean as gas, which is cleaner than oil or coal but still a fossil fuel-powered polluter.
The problem is, you can't really take ALL environmental considerations (mining, transport, washing of the coal and resultant sludge ponds, disposal of the toxins separated from the burnable fuel, transportation of the collected CO2 to storage sites, and CO2 emissions from burning the resultant fuel) into account AND cost at the same time. To actually mitigate ALL the environmental damage would be so expensive that it simply wouldn't make sense to invest in the very expensive plants. It would be cheaper to build solar plants and much cheaper to build wind plants.