Recent developments in wireless systems and sensor technologies have spurred the interest on the design and development of Wireless Body Area Networks (WBANs). The performance of such systems will very much rely on protocols capable of dealing with the stringent QoS requirements of the end applications. Towards this end, numerous studies are being carried out aiming to evaluate the IEEE 802.15.6 MAC protocol. In this paper, we undertake the development of a simulation tool enabling the study of the IEEE 802.15.6 slotted-Aloha MAC with power control. Our study particularly focuses on a multiuser environment where the monitoring devices of a number of users will communicate with a central hub. Such scenario will typically describe the operating conditions of a nursing unit.