A compact modular isothermal reactor for converting a feedstock such as methanol into a fuel usable in power generation systems, such as electrochemical fuel cells, is provided. The reactor includes a sealing plate, a baffle plate having heat transfer surfaces extending toward the sealing plate, and a housing having an interior substantially circumscribing the heat transfer surfaces The cooperating surfaces of the sealing plate, baffle plate, heat transfer surfaces and housing interior define a labyrinthine channel for containing a suitable quantity of solid catalyst and through which feedstock flows. The cooperating surfaces of a fluid flow plate and the baffle plate define a flow passage for carrying a thermal fluid. Heat is transferred from the thermal fluid to the feedstock by the heat transfer surfaces. In two alternative designs, thermally conductive pins extending from the baffle plate or a pair of spaced folded thermally conductive plates transfer heat to the interior of a catalyst containment vessel. In an optional heat exchange section, the feedstock is preheated before being directed to the reactor.