Balancing between energy sources and end-users with different energy consumption behaviors is becoming more challenging due to the increased influence of variable renewable power and environmental limits on the use of fossil fuels. Power-To-Gas (P2G) technologies offer a great potential to store energy from renewable energy generation sources in gas pipeline networks that would otherwise be curtailed. Modeling gas networks in an interconnected energy system is important due to gas quality changes, different end-users, and network constraints. Therefore, it is crucial that the simulations produce accurate results quickly. This project seeks to provide novel methods to model gas networks considering gas quality changes when injecting green gases such as hydrogen and Synthetic Natural Gas (SNG). Two different case studies have been modeled and verified by SAInt results; a simplified Irish gas network with 13 nodes and a more realistic Ireland's gas network with 109 nodes.