From the Shea Stadium / Citi Field FAQ on MLB.com:
What are the plans for tearing down Shea Stadium, and how will that be done?I do not envy those construction workers. I wonder how many "pieces" of Shea will end up on eBay.Fifteen days after the last game, the Mets will turn over the stadium keys to the demolition crew, which will immediately begin to tear down the 45-year-old building. Since New York laws now preclude implosion of buildings or the use of a wrecking ball (you've seen all those old pictures of that big ball knocking down the Polo Grounds and Ebbets Field), the demolition will be accomplished piece-by-piece, beginning with knocking out all the concrete and then taking down the steel skeleton like a decomposing Erector Set.