A microchannel reactor block, with process and coolant channel planes arranged in alternate fashion and cross flow mode was considered for Fischer-Tropsch(FT) synthesis. Computational Fluid Dynamic (CFD) simulation of heat transfer in the microchannel block was carried out to gain insights on the complex thermal behavior. Cooling oil, subcooled water and saturated water were chosen as coolants to simulate the heat transfer phenomena. The study revealed significant enhancement in heat transfer across the channel walls due to wall boiling condition which occures in case of saturated water as coolant. Consequently, near isothermal condition was able to achieve by adjusting saturated water flow rate. Scenario for partial boiling, with saturated water as coolant, was investigated. Simulations were carried out to study effects of different conditions of heat generation and coolant flow rate on reactor performance parameters.