He did so, in part, the way millions of other Americans do — with the tax benefits of an individual retirement account. But he was able to turbocharge the impact of those advantages and other tax breaks in his severance package from Bain in a way that few but the country’s super-rich can ever hope to do.

Romney’s former colleagues say his retirement package is a well-justified reward for a chief executive who built Bain from scratch in 1984 into a financial powerhouse that backed business successes such as Staples and the Sports Authority.

The structure and tax treatment of his retirement, including the IRA, was legally sound and appropriate, they say, adding that he has earned less money over his career than some other top private-equity executives, who earned billions of dollars during the same period.

Details of Romney’s retirement assets are somewhat vague because he has released only one year of full tax returns and declined to provide additional specifics about his personal finances. But interviews with Bain executives and accounting professionals show that he was able to take advantage of tax benefits in innovative ways open only to a narrow slice of extremely affluent people — mostly those who work in private-equity firms and other investment partnerships.

His severance package, for instance, allowed him to continue sharing in the profits of the company as if he were still a partner managing it, according to his 2010 tax return and interviews with present and former Bain executives. And because he benefited from the firm’s investments as if he were an active Bain partner, he paid taxes at a lower rate on these earnings than if they were treated as ordinary retirement income. Romney negotiated the package when he was leaving the firm, Bain executives said, while he set up his IRA long before.

IRAs were established by Congress nearly 40 years ago to help people save for their retirement. Under the law today, individuals may contribute up to $5,000 per year and employers may contribute up to $50,000 a year to an employer-sponsored IRA. The money is invested, and the investments grow tax-free until retirement. There is no limit on how much money an IRA can earn tax-free.

What determines an IRA’s growth is the performance of the investments, and Bain enabled Romney, its other employees and its partners to score big on that front. It was not uncommon for senior Bain executives to accrue IRAs valued at tens of millions of dollars, according to former and present company employees, by buying into Bain investments at very low prices and then reinvesting the returns in other low-priced Bain investments after the initial investments appreciated.