Sewage wastewater heat exchanger (SWHE) has a significant role in the performance of sewage wastewater sourced heat pump (SWSHP) system as it provides to transfer the energy of wastewater to intermediary fluid or working fluid. Thus, a theoretical analysis of the SWSHP system was carried out to investigate the effects of SWHE design parameters on the system’s performance. For this purpose, a simulation program based on the proposed mathematical model of the SWSHP system was developed in MATLAB. Afterward, the indirect type SWSHP system that can meet 50 kW heating load was theoretically designed. The influences of SW temperature, its mass flow rate, the inner diameter of the heat exchanger tube, and intermediary fluid mass flow rate on the performance of the designed SWSHP system were analyzed. The results indicate that variation of SW temperature affects the COPsys more than the variation of SW mass flow rate. Considering the ranges of parameters investigated, the COPsys raises from 2.56 to 4.51 and 2.89 to 4.27 with the variations of SW temperature and SW flow rate, respectively. Moreover, an increase in the intermediary fluid mass flow rate provides an improvement on the COPsys and COPunit. However, SWSHP performance is adversely affected by the increasing value of the inner diameter of the tubes. As a result, small changes in the design parameters of the SWHE directly affect the system performance and system operating conditions.