Synthesis gas, fuel gas, or reducing gas is produced by the noncatalytic partial oxidation of a sulfur-containing liquid hydrocarbonaceous fuel or a slurry of sulfur-containing solid carbonaceous fuel with a free-oxygen containing gas in the free-flow reaction zone of a refractory lined gas generator at an autogenous temperature in the range of about 1900.degree. F. to 2900.degree. F. and above the ash-fusion temperature of the slag formed in the reaction zone, so that about 75 to 95 weight percent of the carbon in the fuel feed to the reaction zone is converted into carbon oxides. At least a portion of the hot effluent gas stream from the reaction zone is passed through a free-flow radiant cooler in admixture with an iron-containing additive. In the radiant cooler at least a portion of the sulfur-containing gases e.g. H.sub.2 S and COS react with the iron-containing additive to produce particulate matter comprising iron oxysulfide. Further, a portion of this newly formed particulate matter and/or the iron-containing additive combine with molten slag and/or ash in the hot raw gas stream passing through the radiant gas cooler. The slag produced thereby has a reduced ash fusion temperature and a reduced viscosity. The remainder of the newly formed particulate matter comprising iron oxysulfide and particulate carbon are entrained in the effluent gas stream leaving the radiant syngas cooler and are separated from the effluent gas stream and optionally recycled to the partial oxidation reaction zone in admixture with fresh fuel feed.