This research aimed to study the relevant work relating to environmental evaluation associated with global warming, acidification, eutrophication, and human toxicity from the 120 MW combined-cycle cogeneration power plant through use of a life cycle assessment. The functional units of study were one kWh of electricity and one ton of steam production. The system boundary of this study comprised unit processes related to a gas turbine power plant and thermal power plant using natural gas. Input data including natural gas and demineralized water in the gas turbine process, while oxygen scavenger chemicals such as neutralizing amine and phosphate were included in the steam turbine process. We found that global warming potential and acidification potential came primarily from gas combustion during the production process, while transportation posted a minor contribution, while eutrophication potential and human toxicity caused by NaOCl was 10%. The feasible ways to reduce environmental impacts included cooling down the air temperature prior to being fed to the compressor using the evaporative method and the fogging method. The results found that the fogging method was proven to reduce global warming potential more significantly than the other method. On the other hand, the evaporative method was more effective in terms of acidification, eutrophication, and human toxicity reduction.