The specification design of this heat exchanger has been taken from a real project executed by Clough. The process data given matches the real application exactly and was used to design the heat exchanger. The Regeneration Gas Heater was manufactured in Malaysia, and was shipped to a fabrication yard in Thailand for installation within a pre-assembled module. This paper illustrates fully thermal and mechanical design for this heat exchanger. Here, the redesign takes place to improve the design by reselect different parameters which can enhance the heat transfer through the exchanger. As well as, new correlation is developed to predict Nusselt number for tube side which reduces the error percent with Kern's method from 15.25% to 12.64% based on simulation data. Where, the proposed and Kern's correlations compared against experimental data to show that the proposed correlation is quite accurate. After many iterations, the new design suggested that four tubes passes need to be used with 1.38 m tube length. In addition, the tube arrangement chosen to be square type with 0.9 m shell diameter. With these parameters, the exchanger achieves high enough heat transfer coefficient and the pressure drop within specification.