Apparatus for the conversion of solid fuels and solid organic waste materials by high temperature gasification into gaseous fuel called "producer gas." The apparatus comprises a stacked two-section gasifier defining sequentially descending drying, distillation, oxidation and reduction reaction zones through which a column of the solid fuel descends during its conversion to the gaseous fuel. The lower reactor section is of double-shell construction and defines the lower oxidation and reduction reaction zones. Means are provided for drawing air into the oxidation zone for burning reaction with carbonized fuel passing therethrough and for thereafter drawing reaction gases downwardly through the lower reduction zone of the gasifier and then through the annular space defined by the double-shell structure of the lower section in indirect counter-current heat exchange relationship with the fuel column portion in the oxidation and reduction zones. The inner shell element of the lower section is arranged in hanging manner within its associated outer shell element to allow expansion of the double-shell structure under the high temperature conditions experienced by the gasifier without harmful stress build-up in the apparatus. The inner shell element supports within its lower portion two stacked funnel-shaped transition pieces which form a series of throat-like constrictions for supporting the fuel column in the gasifier and which cause localized increase in the velocity of the gases passing downwardly therethrough and leaving the reduction zone of the reactor.