By John S. McClenahen How does an executive successfully succeed a management legend? By quickly establishing and communicating his or her own agenda, say U.S. and European corporate succession experts. For Jeffrey R. Immelt, the newly designated president and chairman-elect of General Electric Co., Fairfield, Conn., succeeding the legendary John F. "Jack" Welch Jr. will be complicated by the fact that Welch plans to remain chairman and CEO for another year and oversee the pending $45 billion merger of Honeywell International Inc. into GE. And that won't be the only challenge Immelt will face. Wall Street securities analysts, with an intense focus on short-term results, will expect Immelt to continue to post Welch-quality financial performance. GE's market capitalization now is nearly $500 billion, slightly less than Canada's entire GDP. Indeed, based on nominal GDP figures calculated by Merrill Lynch & Co., New York, if GE were a country, it would have the 11th largest economy in the world -- ahead of such places as Russia, Mexico, India, the Netherlands, Australia, Argentina, South Korea, Switzerland, and Taiwan. "Jeff Immelt is a natural leader, and ideally suited to lead GE for many years," Welch says. "He brings a keen strategic intellect, a cutting-edge technical background, strong leadership characteristics, and a unique set of team-building skills." Immelt, 44-years-old, most recently was the highly regarded and innovative president and CEO of GE Medical Systems, a $7 billion business based in Waukesha, Wis. He has a BS in applied mathematics from Dartmouth College and an MBA from Harvard Business School. Like Welch, Immelt is likely to have little patience with managers who fail to make their numbers.