The effect of a fast exothermic surface reaction on the fluid flow and on heat transfer between bulk gas phase, and the catalytic wall in a monolithic reactor has been studied by means of a 2-D mathematical model. The radial profiles of temperature, concentration, and velocity in the ignition region show that the perturbation induced by the reaction is qualitatively comparable to the entrance effects. From the standpoint of heat transfer efficiency the channel may be divided into two zones, the first dominated by the entrance effects and the second dominated by the reaction effects. In the first zone the interphase heat transfer resembles the constant wall heat flux (Nu(H)), while after ignition resembles the constant wall temperature (Nu(T)) only once the perturbation generated by the reaction ignition has extinguished. It is, hence, showed that the Nu number may be calculated as the interpolation between Nu(H), and a modified Nusselt number Nu(ad), which is defined as a function of the difference between the adiabatic temperature and gas bulk temperature rather than wall-gas temperature difference. It is anchored to the ignition location, depends on the operating and kinetic parameters and asymptotically tends to NUT. (c) 2005 American Institute of Chemical Engineers.