The site will provide enough renewable energy to supply around 6,000 households and businesses with electricity.
Northern Ireland energy minister Arlene Foster said the opening was "a very important milestone".
She added: "The Giant's Park installation is the largest non-wind renewable generating station to come into operation in Northern Ireland and only the second to produce electricity from landfill gas.
Councillor Naomi Long, the Lord Mayor of Belfast, claimed the plant would be of "enormous environmental and economic benefit" to the city.
The Carbon Disclosure Project recently named energy supplier Centrica as the UK's top performing utility company.
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