A gasification/melting facility that can operate up to 10 bar and 1,550 ℃ with a maximum 1 ton/day capacity was developed for liquid and slurry-type combustible wastes. The main focus of the system development was minimal use of expensive fuel for maintaining the reaction temperature by replacing it with cheap waste oil for energy input. The carbon conversion obtained was 97% while the cold gas efficiency reached 77.6% for the refined waste oil. When the feed was refined oil mixed with fly ash from a municipal waste incinerator, the carbon conversion and cold gas efficiency were 93% and 71.9%, respectively, with a slag conversion ratio of 0.93. The slag produced from fly ash exhibited environmentally acceptable heavy-metal leaching values and thus can be applicable as road material and for other purposes. The optimal O₂/feed ratio was 0.9-1.0 when only the refined waste oil was gasified, whereas the O₂/feed ratio had to be higher than 1.2 when fly ash was mixed. In addition, data showed that gasifier temperature can be estimated by on-line methane concentration measurements.