Environmental issues, for example, the expanded air pollutant emissions from ships are progressively affecting the operation of ships. Therefore, International Maritime Organization (IMO) has adopted many goals to decarbonizing the shipping industry by at least 40% by 2030. Marine fuels play a major role in these goals because of the emissions resulting from the combustion process. Therefore, the present research proposes to convert the conventional engine operated by marine diesel oil (MDO) to a dual-fuel engine operated by either natural gas (NG) or methanol. As a case study, A15-class container ship is investigated. The results showed that the dual-fuel engine operated with (98.5% NG and 1.5% MDO) will reduce CO2, SOx, and NOx emissions by 28%, 98% and 85%, respectively when compared with their values for conventional diesel engine. On the other hand, the reduction percentages reach to 7%, 95% and 80% when using a dual-fuel engine operated with (95% Methanol and 5% MDO), respectively. The proposed dual-fuel engines operated by either NG and methanol will improve the ship energy efficiency index by 26% and 7%, respectively.