This dissertation proposes and implements a long-term capacity expansion model for the co-optimization of electric and natural gas infrastructures. It allows to determine the required investments in generation units, transmission lines and pipelines for meeting future demands, while representing electricity and natural gas flows using steady state equations. A Mixed Integer Nonlinear Programming problem is developed, from which a linearized version is derived. A twenty six node integrated electric-gas system for the Eastern Region of the United States is used to demonstrate the model's capabilities. Results show that the model provides an accurate operational representation of the integrated system, and therefore, enhances the expansion planning process.