Abstract: The apparatus of U.S. Pat. No. 3,988,123 is improved by constructing the heat recovery stage as a relatively elongated water-wall type of heat exchanger within a pressure shell, rather than of shell and tube type. The heat exchanger is advantageously formed from contiguous, externally finned, vertical tubes that are rigidly joined together to form a laterally closed, heat recovery chamber, the fin portion of one being welded along its tip to the tube portion of another adjoining such fin portion. The heat exchanger tubes preferably continue vertically throughout substantially the entire height of the gasification reaction i.e. combustion chamber of the first stage of the apparatus, completely eliminating the intermediate quench stage of the patented apparatus and serving to recover heat from the combustion stage as well as from the heat recovery stage by a heat exchange fluid, such as water, flowing through the tubes.