Low sulfur, iron and calcium containing solid carbonaceous fuel having a high melting point ash is mixed with a sufficient amount of iron sulfide containing material or supplemental iron and sulfur containing reactant materials that react in the reaction zone to produce iron and sulfur containing compounds. The mixture of materials is reacted in a free-flow gasifier by partial oxidation to produce a raw gas stream of synthesis gas, reducing gas or fuel gas, containing entrained particulate matter including molten slag. By the process, sufficient iron and sulfur are in the reaction zone so that the melting point of the fuel ash is decreased, and the mole ratio H.sub.2 S/H.sub.2 +CO in the raw gas stream is greater than 0.01. Further, the amount of iron in the molten ash is greater than 10.0 wt. %, and preferably at least 14 wt. % of the molten ash. By this means, the fluid temperature of the molten ash entrained in the raw gas stream is reduced at least 100.degree. F. less than the fluid temperature of the fuel ash without the addition of iron and sulfur, and the gasifier may be operated in the molten slagging mode at a lower temperature. Further, coal wash tailings containing iron sulfides may be disposed of as a portion of the feed to the gasifier in an environmentally acceptable manner; and useful by-product sulfur may be recovered.