The refractory lined reaction zone of a free flow vertical gas generator for the production of raw synthesis gas, reducing gas, or fuel gas by the partial oxidation of an ash-containing solid carbonaceous fuel or a high-metal-containing liquid hydrocarbonaceous fuel is preheated and deslagged by first heating the slag and ash layer on the surface of the refractory lining to a temperature in the range of about 1800.degree.-2000.degree. F. by means of an annular-type elongated burner whose downstream tip is located near the upstream roof of the reaction zone. The tip of the burner is then lowered along the central longitudinal axis of the reaction chamber to above the bottom central outlet passage of the reaction chamber, and the slag and ash layer is heated to their melting point, or above. The molten slag at the bottom of the reaction chamber and the flue gas pass out through the bottom central outlet. The burner is then slowly raised vertically to the initial position while simultaneously melting successive portions of the slag and ash layer on the refractory walls. The molten slag flows down the walls and bottom of the reaction chamber and out through the bottom central outlet passage.