Several different designs for a header type were numerically studied to achieve uniform distribution of gas phase flow in the header of a shell-and-tube heat exchanger. The different geometries included the position and shape of the inlet nozzle, number of outlet tubes, and length. In numerical calculations, the k-epsilon realizable turbulent model was employed. Standard deviation was used to evaluate the uniformity of the velocity distribution among the whole outlets of the header. As a result, flow patterns in the header could be visualized by using post-processing of numerical results. The uniformity of flow distribution increased with header length, whereas it decreased with gas flow rate. Furthermore, the optimum position and shape of the inlet nozzle could be proposed for a uniform distribution of a 1.3 m-length header, the very same used for the heat exchange of the commercially viable allyl chloride process.