Printed circuit heat exchangers (PCHEs) are promising to be employed in high-temperature gascooled reactors (HTGRs) due to their compactness and intrinsic characteristics of capable of providing high-temperature and high-pressure heat for industrial applications. In our previous study, a reduced-scale zigzag-channel PCHE was fabricated out of Alloy 617 and its heat transfer and pressure drop characteristics were investigated experimentally in a high-temperature helium test facility. In our current study, a computational fluid dynamics (CFD) code, STAR-CCM+, was used to simulate the thermalhydraulic performance of the fabricated PCHE with a simplified geometry model. Comparisons between the experimental data and the CFD simulations showed some discrepancies in the pressure drop and heat transfer results, which may be caused by the use of different thermal boundary conditions in the simulations from those in the experiments. The simplified heat exchanger simulation model was divided into eight segments to identify the thermal boundary conditions for the zigzag-channel PCHE. The temperature and heat flux distributions along the fluid flow direction in the heat exchanger for each segment were obtained. It was observed that the temperatures were not constant along the azimuthal direction of a cross section of the flow channel and the helium temperature distribution for each segment presented a wavy shape. However, the global helium temperature distribution along the entire flow channel was approximately linear. For the heat flux distributions, although they were significantly different at different segments, the trend for the heat flux for each segment along the fluid flow direction was similar.