Synthesis gas is produced by the partial oxidation of a feedstock comprising sulfur-containing petroleum coke and/or liquid hydrocarbonaceous fuel and having ashes that contain nickel and a comparatively low level of silicon. The production of toxic nickel subsulfide (Ni.sub.3 S.sub.2) in the slag is prevented, and there is substantially no increase in the mole ratio H.sub.2 S+COS/H.sub.2 +CO in the raw product gas stream by introducing an additive comprising about 2 to 100 wt. % of a sulfur-containing material and any remainder comprising a supplemental iron-containing material into the reaction zone along with the feed. The additive combines with substantially all of the nickel constituents and most of the iron constituents and sulfur found in the feedstock to produce molten slag substantially comprising a fluid sulfide phase and a relatively minor amount of an Fe, Ni alloy phase. In another embodiment, a minor amount of a calcium compound is included with the additive at startup to reduce the softening temperature of the iron-containing addition agent. In still another embodiment, the additive is uniformly dispersed in sulfur-containing petroleum coke having a nickel-containing ash. By this method molten slag free from toxic Ni.sub.3 S.sub.2 and having a reduced viscosity may be readily removed from the gas generator at a lower temperature. Further, the life of the refractory lining is extended.