A fuel conversion reactor includes a shell-and-tube heat exchanger for controlling the temperature of a hot gaseous mixture produced by catalytic or non-catalytic reaction of a fuel with a gaseous fluid, and for controlling the temperature of the gaseous fluid and/or the fuel prior to the reaction. The reactor is either a catalytic or non-catalytic burner, or a fuel reformer for converting a fuel to hydrogen. A preferred reactor includes an outer shell having first and second ends and an inner surface, a primary inner shell extending into the outer shell, the primary inner shell defining a heat exchanging chamber and having primary and secondary ends, and a secondary inner shell having a first end located adjacent the secondary end of the primary inner shell. One or more outlet apertures are formed between the two inner shells for passage of the gaseous fluid out of the heat exchanging chamber. There are also a plurality of heat exchange tubes extending through the heat exchanging chamber between first and second tube sheets and connected to same. The first tube sheet is mounted in the primary inner shell while the second tube sheet is connected to the secondary inner shell. The tubes form passages for flow of the hot gaseous mixture in heat exchange contact with the gaseous fluid through the heat exchanging chamber, thereby preheating the gaseous fluid prior to reaction with the fuel. The adjacent ends of the inner shells form a disconnected joint and the secondary inner shell is free to move relative to the primary inner shell upon thermal expansion of the tubes.