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 a first free-flow reaction zone R.sub.1 located in a refractory lined gas generator at an autogenous temperature in the range of about 190.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 first 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 a calcium-containing additive. An equilibrium oxygen concentration with a partial pressure which is less than about 10.sup.-12 atmosphere is provided in the gas phase in the first and second reaction zones. 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 produced in R.sub.1 and R.sub.2 e.g. H.sub.2 S and COS react at high temperature with the calcium-containing additive to produce particulate matter comprising calcium sulfide. Further, a portion of this newly formed particulate matter and/or the calcium-containing additive combine with slag and/or ash in the hot raw gas stream passing through the second gas cooler. Fly-ash is produced thereby having an increased ash softening temperature. The gas stream discharged from the second reaction zone contains a reduced amount of sulfur-containing gases, and increased amounts of H.sub.2 + carbon oxides and calcium sulfide-containing particulate matter.