Computational fluid dynamic (CFD) simulation of heat transfer in a microchannel reactor block for low temperature Fischer-Tropsch (FT) synthesis was considered. Heat generation profiles for different operating conditions (GHSV 5000 h-1; catalyst loading 60%-120%, where 100% loading equals 1060 kg/m3 of cobalt based catalyst from Oxford Catalyst Ltd.) were obtained from a single channel model. Simulations on a reactor block quantified the effects of three coolant types: cooling oil (Merlotherm SH), subcooled water and saturated water, on reactor temperature, and also evaluated the effect of wall boiling conditions. At process conditions of GHSV 5000 h-1 and catalyst loading of 120%, predicted temperature gradients along channel length were 32, 17 and 12 K for cooling oil, subcooled water and saturated water, respectively. A modified reactor block showed improved thermal performance as well as heat transfer enhancement due to wall boiling conditions.