Great amounts of renewable energy are installed into power systems at state, regional and national level, often due to fulfill legislated mandates or renewable portfolio standards (RPS). While renewable energy is a means for reducing reliance on fossil fuels and decreasing greenhouse gas emissions, it is increasingly evident that there is need for flexible thermal fleet to help balance the renewables. The primary fuel considered for new builds is natural gas, and the default technology to meet capacity and flexibility needs is gas turbines in simple or combined cycle. In this work we show the substantial system benefits of increased flexibility and improved dynamic dispatch capability. This is achieved by exchanging traditional gas turbine based plants in the planning process to gas-fired combustion engine plants.