We present a simulation study of the countrate performance of a PET scanner with partial collimation. In this study, partial collimation is achieved by removal of every other septum from the standard 2D septa set for the GE Advance PET scanner. System behavior is evaluated with a photon tracking simulation package (SimSET) and calibrated to measured data for 2D and fully-3D acquisition modes using the NEMA NU-2 countrate phantom. Results are evaluated in terms of true, scattered, and random coincidences and noise equivalent counts (NEC) regarding both counts per image plane and total counts as a function of activity. Our results show a good agreement between the measured and simulated count rates for the Advance PET scanner for the 2D (full collimation) and fully-3D (no collimation) acquisition modes, increasing our confidence in the predicted countrate results for the partial collimation mode. The latter results in a countrate performance intermediate between the 2D and fully-3D acquisition modes and yields a more favorable countrate performance for clinical activity levels.