This paper presents the results of a study on the influence of the fuel gas composition on the thermodynamic performance of a typical heavy duty gas turbine, followed by an emissions prediction using a chemical reactor network which, in its turn, uses one of the most developed chemical kinetic mechanism for the natural gas combustion. The first part of this work regards a statistical analysis of a time series which shows the fuel gas composition variation along a certain period, revealing the typical concentrations of the gas species. Based on the curves provided by the statistics, several gas compositions were proposed and applied to a thermodynamic model in order to evaluate the impact of the species composition of the natural gas on the gas turbine heat rate and power. Afterwards, several approaches of chemical reactor network were considered in order to choose the methodology that better predicts the temperature and CO and NOx formation within a gas turbine combustor operated at base load regime. Finally, it was studied the influence of the natural gas composition variation over the aforementioned thermochemical properties at the exit of the combustion chamber when considering both operation regime and combustor's natural gas composition.