The Q-Chem II plant is expected to add more than 75 per cent to the output capacity of Qatar Chemical Company, the plant's owner, the company said.
Energy Minister Abdullah al-Attiyah said the plant aims to meet increasing global demand for petrochemicals.
In May, he said gas-rich Qatar plans to boost its annual petrochemicals production to 28 million tonnes by 2014.
Qatar holds the world's third-largest natural gas reserves, estimated at more than 900 trillion cubic feet (25 trillion cubic metres), after Russia and Iran.
A member of the Organisation of Petroleum Exporting Countries, the Gulf state also produces some 800,000 barrels of crude oil per day.
Qatar Chemical Company is a joint venture between Qatar Petroleum Corp (51 per cent) and Chevron Phillips Chemical Co (49 per cent) of the United States.