Blackstone and Thomas DeLeo, Sithe's chief operating officer, confirmed the news.
"We are greatly saddening by Bruce's passing. He was a true visionary, a colleague and a friend. He built the company from the ground up and will be deeply missed," DeLeo said.
The cause of death, sometime in the last 24 hours, was not clear. Wrobel appears to have been in his mid-to-late 50s; he graduated from the Massachusetts Institute of Technology with a bachelor's degree in 1979.
Wrobel co-founded Sithe Global Power, which is responsible for the $900 million Bujagali Hydroelectric Project in Uganda, which dramatically increased electricity in the country. Sithe is owned by Blackstone Group, which initially took an 80 percent stake in the company in 2005 and acquired 99 percent of it in 2011.
In July, Sithe and Blackstone announced that Wrobel would retire from his position as chief executive. No reason was given.
"We are deeply saddened by Bruce's sudden passing. Bruce was a good friend and a true visionary, who had a positive affect on everyone with whom he interacted," said David Foley, chief executive officer of Blackstone Energy Partners. "He will continue to have a lasting impact through the many innovative energy projects he worked tirelessly to develop, and he will be sorely missed by our Blackstone team."
Wrobel also helped found SEACOM, which launched Africa's first broadband submarine cable system in 2009. The project greatly increased the reach and quality of internet service in East and Southern Africa.
More recently, Wrobel was CEO of both Global Alumina Corporation, which worked on mining in Guinea, and Herakles Farms, which had sought to build palm oil farms in Cameroon. Herakles was forced to stop production in May 2013 because of pressure from local residents and environmental activists like Greenpeace and Oakland Institute. Herakles and Global Alumina did not immediately respond to a request for comment.
Wrobel was also active in charity through All for Africa, which he chaired.
Wrobel focused on the power industry throughout his career. He was responsible for the development or acquisitions of more than 50 power plants around the world, according to his public biographies.
-By CNBC's Lawrence Delevingne. Follow him on Twitter @ldelevingne .
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