NOVELTY - The catalytic reformer assembly for reforming hydrocarbon fuel e.g. natural gas to generate a hydrogen-containing reformate fuel, comprises an endothermic reactor (14) including side-by-side tubes (42) for defining parallel flow paths in a first direction through the tubes for reforming a gaseous fuel mixture including the hydrocarbon fuel, a first substrate (48) disposed around the tubes for defining a first heat exchanger for extracting heat from tempering gas when the gas is passed through the first substrate and a second substrate (52) disposed within each of the tubes.    USE - The catalytic reformer assembly is useful for reforming hydrocarbon fuel to generate a hydrogen-containing reformate fuel and useful in a solid oxide fuel cell system (claimed), where the hydrocarbon fuel is natural gas, light distillates, methanol, propane, naphtha, kerosene, gasoline and/or diesel fuel and air and the hydrogen-containing reformate fuel is a gaseous blend of hydrogen, carbon monoxide and nitrogen.    ADVANTAGE - The catalytic reformer assembly is compact, relatively inexpensive and easy to manufacture, has long working life, excellent thermal conductivity and oxidation resistance, and allows easy and ready removal and replacement of the substrates as required from time to time by melting out the braze thus improving the efficiency of the fuel.    DETAILED DESCRIPTION - The catalytic reformer assembly for reforming hydrocarbon fuel e.g. natural gas to generate a hydrogen-containing reformate fuel, comprises an endothermic reactor (14) including side-by-side tubes (42) for defining parallel flow paths in a first direction through the tubes for reforming a gaseous fuel mixture including the hydrocarbon fuel, a first substrate (48) disposed around the tubes for defining a first heat exchanger for extracting heat from tempering gas when the gas is passed through the first substrate and a second substrate (52) disposed within each of the tubes defining a second heat exchanger for imparting heat to the gaseous fuel mixture when the gaseous fuel mixture is passed through the second substrate, and a housing (40) surrounding the tubes and the first substrate defining a flow space to flow the tempering gas through the first substrate. The second substrate is supportive of a catalytic material. A portion of the heat extracted from the tempering gas is conducted by walls of the tubes from the first heat exchanger to the second heat exchanger. The tempering gas is passed through the first substrate in a direction opposite to the first direction defining a counter-flow relationship of the tempering gas to the gaseous mixture. The first and second substrates have a shape of corrugated sheet metal, rolled wire mesh, wire gauze or a metal foam lattice. The gaseous mixture includes water and anode tail gas. The first substrate or the second substrate is attached to the tubes by brazing or welding. The tempering gas is combustor exhaust resulting from combustion of anode tail gas from a fuel cell stack. An INDEPENDENT CLAIM is included for a solid oxide fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a cross-sectional view of a multi-tube reformer reactor.    Reactor (14)    Housing (40)    Tubes (42)    First substrate (48)    Second substrate. (52)