When a heat exchange device and its fluid-mover partner are modeled simultaneously, as a single system, the predicted performance of the device can be substantially different from modeling the heat exchange device separately. This conclusion is based on realistic models of the underlying heat transfer and fluid flow phenomena, on the rotational motions occurring within the fluid mover, and on the geometry of both the heat transfer device and the fluid mover. Here, an unsteady three-dimensional model was implemented by numerical simulation as a case study of a thermal management system that is frequently encountered with electronic equipment. As a comparison case, the same thermal management system was evaluated using a common design method where the fluid mover (fan) in the system is removed and replaced by a prescribed magnitude and distribution of fluid flow (the fan curve). It was found that the local and overall heat transfer and the fluid flow delivered to the heat exchanger were significantly overpredicted by the simplified design model relative to those from a more rigorous approach. A broad conclusion of the present work is that better results can be obtained when the heat exchange device and the fluid mover are modeled simultaneously.