An understanding of complex interactions among energy, environmental, and economic activities is important for making decisions to support sustainable economic development and environmental protection. Energy models at global, national, and provincial levels are effective tools used for examining these interactions. However, these models are inadequate for a city-cluster jurisdiction such as the Toronto-Niagara Region (TNR), Canada, which has unique economic and energy characteristics. The objective of this study is to develop a City-cluster Energy Systems Planning Model (CCEM) and apply it to the TNR as a case study. It is demonstrated that the model can be effectively used for supporting energy planning, environmental management, and greenhouse gas (GHG) reduction in a city-cluster jurisdiction.