In this paper the exergy-based thermocharacterization method is used to optimize a small scale downdraft gasifier. The optimization model is based on a Beta-parameter that is used to estimate the variation of exergy change in a component due to either intrinsic or induced malfunctions. As a novelty, the proposed method performs two analyses independently: one to find the variation of efficiency with respect to the change of the system enthalpy and another with respect to the change of the system entropy. This permits to characterize the thermodynamic performance of any gasifier by means of parametric maps. Hence thermo-characterization premises are deeply revised and adapted to the gasifier. To assess the system performance and determine the irreversibilities of the components, a plant simulator is constructed and a parametric study is performed. According to results, the exergy efficiency of the gasifier can be improved from 49.64% to 57.52%.