As Wall Street firms crank up their machines for issuing new commercial mortgage-backed securities, a big prize is emerging: the business of mall giant General Growth Properties Inc. General Growth, which just emerged from bankruptcy protection, is planning to refinance some of the $15 billion in mortgages it restructured during the Chapter 11 process. While some of that will likely be done through insurance companies, a big chunk is likely to be repackaged by Wall Street firms into CMBS. That is a lot of business considering only $10 billion worth of CMBS has been issued this year as the market has struggled to get back on its feet. During the boom years, CMBS was a financing mainstay of the commercial-real-estate industry, with $230 billion worth of the securities issued in 2007. But new issues ground to a halt during the recession.