This paper provides a first report of a novel radial microchannel reactor (RMR) architecture for catalytic steam reforming of methane to hydrogen and/or synthesis gas. A bench-scale RMR prototype with a channel width of 0.700 mm and wash-coated with 0.030 mm of a Ni-based catalyst is operated at 750 degrees C and 11 bar with feed composition of 25 mol % CH4, balance H2O. Results indicate that volumetric heat fluxes in excess of 150 W cm(-3) are achievable at residence times of 6.8 ms, while volumetric heat fluxes in excess of 100 W cm(-3) are observed at overall thermodynamic efficiencies >70%. Computational fluid dynamic (CFD) simulations of the RMR design are demonstrated for accurately predicting methane steam reformer performance and, subsequently, for rapidly investigating the influence of microchannel dimensions upon RMR performance. Design simulations predict that 20-100% improvements in volumetric heat-transfer rates may be achieved by reducing channel widths from 0.700 to 0.300 mm.