City officials say plans to rebuild the long-vacant old main Post Office in downtown Chicago have been finalized.

In a statement, Mayor Rahm Emanuel announced that 601W Companies has purchased the hulking building that straddles Congress Parkway near the river.

Emanuel says the New York-based company will move forward with a $500 million redevelopment plan, expected to be completed by 2018.

“Today we are taking another significant step towards transforming the Old Main Post Office site into an economic driver for the City of Chicago,” Emanuel said in a statement. “This project will create thousands of jobs and generate new economic opportunities for residents in our neighborhoods, while restoring and reviving an iconic gateway to our city.”

The plan is to convert the building into offices, featuring a three-acre rooftop park.

“601W will reposition the building as an incredibly unique campus allowing companies seeking large, open layouts with plenty of light and air,” 601W’s Mark Karasick said in a statement.

The 85-year-old post office building has been vacant since 1995.