This paper presents the work on developing accurate and computationally efficient PCHE off-design quasi-steady state performance models for both recuperator and pre-cooler in a supercritical CO2 (S-CO2) Brayton cycle respectively to optimize power system operation strategies under off-design conditions. The newly developed model for heat transfer uses an existing Log Mean Temperature Difference (LMTD) method and modifies it to reflect the rapid change of properties in the vicinity of the critical point. In addition, the prediction of frictional pressure drop was implemented to this model. In the performance evaluation, this methodology shows its prediction accuracy to be sufficient. When it comes to the computational resource, the suggested model can accelerate the calculation over two orders of magnitude faster. The developed model is applied to evaluate the performance of the S-CO2 system to establish operation strategies such as inventory control and heater bypass control under off-design conditions.