The integration of different wireless networks to provide seamless and continuous network access services is a major issue in beyond-third-generation (B3G) networks. To avoid modifying the existing protocols in the network nodes, the application-level approach is proposed for heterogeneous network integration. In the application-level integration, a mobility gateway (MG) is introduced to serve as an agent through which users can use different kinds of mobile devices through different kinds of networks to access Internet applications. There are two major tasks in an MG, including content format translation and data tunneling for handoff users. When the service area of an MG is larger, the MG must handle the content format translation for more users, and the computation overhead of an MG increases significantly. On the other hand, when the service area of an MG shrinks, a served user more likely hands off between different MGs, and more network resources are consumed to tunnel the data for handoff users. In this paper, an analytical model is proposed, and simulation experiments are conducted to study how to properly plan the service area of an MG so that better system performance can be achieved.