1. A process for producing fuel gases from at least one non-gaseous hydrocarbonaceous material, using a two-stage reaction apparatus, comprising the following steps, carried out cyclically: (a) introducing anon-gaseous hydrocarbonaceous material into a pyrolysis chamber, comprising a first stage of the apparatus, and pyrolyzing said hydrocarbonaceous material therein so as to produce a primary fuel gas mixture, a pyrolysis liquid, and a first carbonaceous residue; (b) introducing said primary fuel gas mixture and pyrolysis liquid into a second chamber, comprising a second stage of the apparatus and containing a catalyst, and heating said liquid therein, in a substantially non-oxidizing atmosphere, to a temperature of 900° to 1100° C. and substantially above the temperature at which pyrolysis is effected in step (a), so as to produce additional fuel gases and additional solid carbonaceous residue, without substantially altering the composition of said primary fuel gas mixture; (c) withdrawing said primary fuel gas mixture and said additional fuel gas from said second chamber; and (d) introducing air, oxygen, carbon dioxide or steam into each of said chambers to effect reaction with, and at least partial removal of, said carbonaceous residue therein.