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 first free-flow reaction zone located in 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. About 85 to 99 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 second reaction zone R.sub.2 in admixture with a second portion of sulfur-containing fuel and an iron-containing additive. In the second reaction zone the carbon in the second portion of fuel, unconverted fuel and particulate matter from R.sub.1 react with H.sub.2 O and/or CO.sub.2 to produce supplemental H.sub.2 and/or carbon oxides. Further, 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 second 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 second reaction zone and are separated from the effluent gas stream and optionally recycled to the partial oxidation reaction zone in admixture with fresh fuel feed.