A method is disclosed for converting waste organic materials into an innocuous product gas. The method comprises maintaining, in a pressure vessel, in the absence of oxygen, at a temperature of 250.degree. C. to 500.degree. C. and a pressure of at least 50 atmospheres, a fluid organic waste material, water, and a catalyst consisting essentially of reduced nickel in an amount sufficient to catalyze a reaction of the organic waste material to produce an innocuous product gas composed primarily of methane and carbon dioxide. The methane in the product gas may be burned to preheat the organic materials.