Goal of the present activity is the development of a 0D/1D co-simulation environment of a CNG HD SI engine model and a Three-Way Catalyst (TWC) model. The proposed model, built though a connection between GT-Suite and Matlab/Simulink platforms, has been validated in Steady-State and Dynamic conditions to a defined lambda variation and allows to predict formation and conversion phenomena of the main chemical species both in engine-out and tailpipe.