Abstract <p>Heat exchanger unit is one of the most important parts in refineries, to ensure the efficiency of heating and cooling process. The improvement of heat exchange not only gives a good refined product, but also decreases the total cost. In this work, we collected heat exchangers´ data and operating conditions and simulated them in HYSYS program to predict heat exchangers´ performance. The choice of the HYSYS program in the study is due to the accuracy of the simulation environment results, which sometimes reaches 98% between the simulation and the reality in the factory. These results were particularly shocking for low-efficiency heat exchangers and indicate multiple causes that affected the low performance of heat exchangers. The large variation in efficiency of these heat exchangers is due to the accumulation of some muds and sediments contained in crude oil and refinery products and the blockage of some flow patterns due to calcified salts over time, it was not known with this accuracy until after using the HYSYS program to simulate the realistic operating conditions along the path of the crude oil flow in this operating unit. Heat exchanger efficiency variation maps along the production process path gave a description of finding and solving difficulties. The current state of Najaf Refinery unit 2 heat exchangers after this study shows the actual low efficiencies of most exchangers gave the Najaf Refinery management and engineers the decision to make emergency maintenance, cleaning process and replace some of these heat exchangers. The optimum operating conditions and change in connection scheme of heat exchangers can be also determined by HYSYS as a forward step.</p>