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One of the two lines stated producing in July 2019, but no announcement is retrievable for the other line. It is therefore assumed that both lines were in production by the end of 2019. 

Jul 25, 2019 -- ExxonMobil recently announced it has started production on a new high-performance polyethylene line at its Beaumont, Texas polyethylene plant. The expansion increases plant production capacity by 65 percent or 650,000 tons per year, bringing site capacity to nearly 1.7 million tons per year. (Source)