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 interposed 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 sub-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 burner gas stream and the process gas stream flow in opposite directions through the assemblage. A varying heat transfer fin density population is disposed in the burner gas passage so as to control the peak burner wall temperatures encountered during operation of the assemblage. The burner wall peak temperature is preferably no greater than about 1,700.degree. F., and is preferably within about 100.degree. F. of the peak process gas temperature.