The scope of this dissertation is the development of a state observer for Power-to-Gas (PtG) plants. A physical performance model of an innovative reactor concept for different operation conditions has been developed and implemented in Matlab-Simulink. Experiments in a lab-scale PtG plant are used to validate the model; a comparison between the experimental and modelled data is presented. The experiments cover a wide temperature range, different operating pressures and inlet volumetric flows. Furthermore, experimental runs are carried out to determine the experimental value of the kinetic parameters, namely the activation energy (Ea), the pre-exponential factor of the Arrhenius form (A) and an empirical coefficient (n). The state observer predicts the performance of the reactor for a specific operating point and allows to identify a decrease of the reactor performance, such an identification of a system state leads to maintenance or modifications in the operating control.