A method and apparatus for producing clean, sweet, fuel gas for use in a combustion process by processing a waste stream from digestion of lignocellulosic material partially oxidizes the waste stream to form hot gases and molten salts and cools the hot gases and molten salts using a quench liquor to form quenched gas (76) and carbonate liquor (30). Particles are removed from the quenched gas to form a raw fuel gas by subjecting the quenched gas to a multi-step fume reduction process (80) which includes heat extraction (91) from the quenched gas to reduce particulate load and water content of the quenched gas to form a low fume fuel gas. H2S is removed from the low fume fuel gas using an H2S removal process (105) which is more selective for H2S than it is for CO2, the removing step forming clean, sweet, fuel gas and acid gases.