The Waste-to-Energy technology through gasification, pyrolysis, combustion, and digestion has recently turned into an unavoidable alternative for the municipalities in various parts of the World. Besides a considerable reduction in the waste volume, it can generate steam and electricity. In the present work, two techniques (gasification and digestion) are implemented in the Tehran's 3 MW Waste-to-Energy power plant; where these technologies are investigated and compared from the viewpoint of thermodynamics, thermoeconomic, and environmental impacts. Also, multi-objective optimization based on genetic algorithm is applied to find the optimum exergy efficiency and total product unit cost of each model. The input municipal solid waste is converted to a high enthalpy syngas via gasification or biogas via digestion to provide the required heat for steam generation in a Rankine power cycle. The best ranges of the effective parameters in optimal point for both models are obtained from the scattered distribution method. Also, the exergy destruction and the exergetic efficiency of every component is calculated to assess the irreversibilities in the system. Results show that the gasifier and the combustion chamber have the highest exergy destruction in the models (a) and (b). Parametric study and environmental impact analysis indicate that model (b) is the more suitable option from energy, exergy, exergoeconomic, and environmental viewpoints. From the multi-objective optimization results, it is inferred that the exergy efficiency for the model (a) and (b) at the optimum point is 17.98% and 19.02%, respectively while the corresponding total product unit cost is 28.31 $/GJ and 27.68 $/GJ.