A process for upgrading plastic material containing inorganic filler or reinforcement material for use as feedstock in a partial oxidation gas generator for the production of raw synthesis gas, fuel gas, or reducing gas. The plastic material is granulated and partially liquefied by heating in a closed autoclave at a temperature in the range of about 400.degree. F. to 495.degree. F. and a pressure in the range of about 150 psig to 750 psig while the plastic material is in contact with a pumpable hydrocarbonaceous liquid solvent. A pumpable slurry is thereby produced comprising solubilized plastic, unsolubilized plastic, hydrocarbonaceous liquid solvent, separated inorganic material and unseparated inorganic material. After removing the separated inorganic material, the remainder of the slurry is then reacted by partial oxidation to produce said raw synthesis gas, fuel gas or reducing gas.