Shares of NRG Energy Inc. NRG rallied 4.05% to $156.71 Tuesday, on what proved to be an all-around mixed trading session for the stock market, with the S&P 500 Index SPX rising 0.72% to 5,886.55 and the Dow Jones Industrial Average DJIA falling 0.64% to 42,140.43.

The deal, to acquire a portfolio of natural gas generation facilities from LS Power Equity Advisors, will double NRG’s power-generation capacity, the company says.

NRG Energy announced Monday plans to acquire 18 natural gas plants across nine states—including Texas—in a move aimed at powering the state’s rapidly rising energy demand. The $12 billion deal will double the company's power generation capacity,