The application of an integrated micro packed bed reactor-heat exchanger for highly exothermic syngas conversion to dimethyl ether was investigated by numerical simulation. A three-dimensional pseudo-homogeneous model was developed to describe concentration and temperature distributions inside a single reaction slit. The model was adopted at low Reynolds numbers (small catalyst particles and long residence times). The simulation results reveal that the micro packed reaction slit is close to isothermal, and a negligible temperature gradient is predicted also for a doubling of the critical reaction slit dimension, i.e. the reaction slit height. The flow in the reaction slit was modelled by Brinkman's equation and it was found that the pressure drop in wide range of gas velocity was not significant. The reactor simulation showed good agreement with experimental results and could be further developed to assess the potential of microstructured packed bed reactors to intensify large-scale industrial processes.