Synthesis gas is produced by the partial oxidation of a fuel feedstock comprising sulfur-containing petroleum coke and/or heavy liquid hydrocarbonaceous fuel and having a nickel and iron-containing ash in a free-flow refractory lined partial oxidation reaction zone. The production of toxic nickel subsulfide (Ni.sub.3 S.sub.2) in said slag is prevented, and there is a substantial reduction in the concentration of H.sub.2 S+COS in the raw product gas stream by introducing an iron-containing additive into the reaction zone along with the feed. The weight ratio of iron-containing additive to ash in the fuel feedstock is in the range of about 0.5 to 10.0. The weight ratio of iron to nickel in said mixture of iron-containing additive and fuel feedstock is greater than 0.33. The additive combines with at least a portion of the nickel and iron constituents and sulfur found in the feedstock to produce a very fluid sulfide phase of iron and nickel, and an Fe, Ni alloy phase. In another embodiment, a minor amount of a calcium compound is included with the iron-containing additive only at startup to reduce the softening temperature of the iron-containing addition agent. By this method, the molten slag produced is free from toxic Ni.sub.3 S.sub.2 and has a comparatively reduced viscosity. Further, the slag may be readily removed from the gas generator at a lower temperature and may be disposed of without contaminating the environment. Further, the life of the refractory lining is extended.