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A two-step configuration consisting water-cooled and gas-cooled reactors has been developed by Lurgi to produce methanol from natural gas. The gas-cooled reactor, which is a vertical shell and tube heat exchanger, is constructed by fixed tube sheets with the cold gas stream in the tube side to properly cool the reacting gas mixture in the shell side. Under the practical operating condition, increase in the temperature drop is observed. As a result of high temperature decrease occurs at the end of the reactor, formation of gas condensate is observed in the fourth year of operation. In this regard, a computational fluid dynamics (CFD) model was developed to predict the behavior of gas streams in the shell and tube sides in order to find the reasons such phenomenea. The obtained results of the developed model were validated with the real plant data and a good agreement was observed. Then, gas velocity and temperature distributions in the shell and tube sides were presented. The obtained results show that the temperature mal-distribution in the shell side is a result of non-uniform flow distribution in the shell and tube sides. Furthermore, this flow distribution non-uniformity inside the reactor comes from an improper design of the distributer inside the reactor.