Steam reforming of methane in microchannels, embedded in a monolith is numerically modelled. Horizontal heating layers at equal intervals within the monolith are maintained at constant temperature. The channels are coated internally with catalyst to enhance gas-solid heterogeneous reaction. The numerical method combines the analytical solution for heat transfer through a fin, extended to a stack of fins, and the reactive flow of gases through an iterative procedure. The method offers a tool for quick design of a micro-structure, without considering detailed CFD-based model. In addition, the method can be suitably modified to address thermal management in electronic chip. The temperature within a stack between two heating layers drops near the centre of the stack, in case of an endothermic reaction. This drop, signifying the deviation from isothermal behaviour is found more near the heating layer, and tapers off near the centre of the stack. When the feed temperature is significantly less than the temperature of the heating layer, the portion of the reactor, away from the heating layer remains at a substantially lower temperature, particularly when the number of channels between two heating layers is large. Accordingly, the conversions in the individual channels at the outlet are affected. If the channel wall becomes thicker, the drop in fluid temperature away from the heating layer is more. The increase in feed velocity leads to larger drop in temperature and overall conversion. The decrease in thermal conductivity and the increase in number of channels between two heating layers enhance the temperature drop. None of these functionalities appears to be linear. (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.