The Jetty Boil-Off Gas Recovery Project (JBOG) would capture enough gas to power 175,000 cars per year or 300,000 homes, Qatari officials said.
It also allows Qatar, the largest producer of liquefied natural gas (LNG) in the world, to recover a further 29 million cubic feet of gas per year.
Qatari officials said it would save the equivalent of 1.6 million tonnes of CO2 each year.
"It is the largest (environmental) project in Qatar," said Saad Sherida al-Kaabi, the Qatargas chairman.
"It is also the largest in the world, others can confirm to me whether that is right or wrong."
The JBOG project salvages gas normally lost and burnt off in a flare when the fuel is transported onto ships.
Officials say it will result in a 90 percent reduction in flaring at the Ras Laffan port in northern Qatar, the largest LNG export terminal in the world.
Instead of the wasted gas being burnt off, JBOG would collect the fuel and transport it to an area where it is compressed to be ready for use again either as LNG or fuel gas.
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LNG is gas cooled to -160 degrees Celsius until it turns into a liquid and can be transported by boat.
Kaabi said the scheme was driven by environmental concerns not economic factors, despite predictions that resource-rich Qatar's dominant position in the LNG market could come under increasing threat from competitors such as the United States or Australia in the next few years.
"This is nothing to do with it," said Kaabi. "It's a purely environmental project."
The scheme has been operational since October and has recovered gas from over 500 ships.
It was officially inaugurated on Tuesday by a number of high-ranking Qatari officials, including Prime Minister Sheikh Abdullah bin Nasser bin Khalifa al-Thani.
Kaabi added that there were no industrial accidents or man hours lost during the construction of the JBOG plant.
"There's a lot of talk about Qatar and the welfare of workers. I think we have shown how much care we have taken at this complex," he said.
Some 3,500 workers and 22 million man hours went into the construction of the plant.
Qatargas operates JBOG on behalf of Qatar Petroleum and RasGas.