A fuel gas reformer assemblage for use in a fuel cell power plant is formed from a composite plate assembly which includes spaced-apart divider plates with columns of individual gas passages. The reformer assemblage is constructed from a series of repeating sub-assemblies, each of which includes a core of separate regenerator/heat exchanger gas passages. The core in each sub-assembly is sandwiched between a pair of reformer gas passage skins, which complete the assembly. Adjacent reformer gas/regenerator/reformer gas passage sub-assemblies in the composite plate assembly are separated from each other by burner gas passages. The regenerator/heat exchanger gas passages and the reformer gas passages in each sub-assembly are connected by gas flow reversing manifolds which form a part of each sub-assembly. The fuel gases flow in one end of the assemblage, through the reformer gas passages, and then reverse their direction of flow in the return manifolds so as to exit the reformer assemblage through the regenerator gas flow passages. The burner gases flow in one end of the reformer assemblage and out the other end. The walls of the burner and reformer gas flow passages are selectively catalyzed after the assemblage has been constructed.