The ability to access information while on the move has become a necessity for most business users. The 3G cellular networks, like UMTS, promise to offer always on, ubiquitous connectivity to users with relatively high mobility, but with relatively low data rate. On the contrary WLANs offer much higher data rates but with lower mobility coverage. The complementary characteristics of the 3G and WLAN networks has attracted the attention of 3G network operators, created the need, and make it attractive to integrate these two technologies. In , we have proposed an integration architecture for UMTS and WLAN networks that allows mobile nodes to maintain data connection through WLAN and voice connection through UMTS in parallel, and facilitates handover of data connections during the vertical handover. This paper presents an OPNET model of the integrated architecture. The model captures the signaling and transmission behavior of the GPRS and 802.11 WLAN networks, mobility management and intersystem handover procedures. The model serves two purposes. First, it verifies the feasibility of the integration architecture, and secondly provides a mean to study the performance of the integration architecture and collect performance metrics, such as handover delay, and number of packets dropped during handover.