I've mounted a few desultory searches for the source of the efficiency loss numbers, and not found them.  How do we know, f'rex, that an IGCC plant would actually become less efficient than a PCC plant with recovery?

An IGCC plant could become far more efficient through the use of a few recent but simple technologies.  Take, for example, Acrion's CO2 Wash process.  This is a fractional distillation system designed to remove CO₂ and contaminants from landfill gas, but it would work equally well to remove CO₂, H₂S, COS, H₂O and the like from a syngas stream.  This leaves mainly H₂, CH₄ and CO.  This in turn could be fractionally distilled (a la air separation plants) to remove the hydrogen.  Carbon monoxide and methane go to solid-oxide fuel cells ($500/kW sometime soon), with the spent fuel gas sent to sequestration.  The air stream for the SOFC's is the combustion air for the gas turbine of an IGCC plant, heated by the hydrogen after exiting the SOFCs (to reach optimum turbine-inlet temperature).

I haven't taken this concept apart far enough to calculate a net efficiency for it, but my SWAG is it ought to be well over 50%.  Nearly all of the carbon, plus 99.9%+ of the sulfur and all the condensible pollutants such as mercury, will be removed from the combustion fuel stream in the two separation steps.  Energy supply for the separation is mostly as low-pressure steam to run the refrigeration system for the first distillation.

All we need is for the sub-$500/kW SOFC to come through, as Delphi and others have claimed they can do.  The rest is mostly re-plumbing a gas turbine.