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The ignition delay times of CH4/C2H6/C3H8 mixtures representative of an average natural gas composition have been measured in a rapid compression machine (RCM) at the University of Lille. The pressure at the end of compression (EOC) varied from 13 to 21 bar and the core gas temperature ranged from 850 to 925 K. Zero-dimensional modelling starting from the EOC was used to reproduce the experimental ignition delay times taking into account heat losses during the preignition phase. The experimental database served as basis for the development of a reaction mechanism suitable for HCCI like autoignition simulations on a stationary co-generation engine with a prechamber, which is under development at the laboratory. Different mechanisms for natural gas oxidation and combustion have been tested and their low temperature simulation ability investigated, showing difficulties to properly reproduce the low temperature ignition delay times. Starting from the GRI3.0 mechanism with an additional submodule improving the low temperature chemistry representation, a sensitivity analysis was performed to determine the most influential reactions. The rate constants of these reactions were then optimised within their range of uncertainty using a multi- objective strategy. The resulting optimised mechanism led to a strong improvement of the agreement between simulation and experimental RCM data. The optimised mechanism was also tested on experimental shock tube data between 900 and 1250 K and gave satisfying results within the temperature range where the optimisation was performed on. Therefore, the applied optimisation technique showed its efficiency.