If you’re looking to convert it into a raw HTML template, be prepared to dig in. You’ll have to remove all of the WP functions and PHP. Depending on the complexity of the theme and your knowledge of PHP / WordPress, could amount to a ton of hours to get what you want.
Explaining what you intend to convert it to and use it for would definitely help us answer your question.
You could just contact the author of the theme and ask for a HTML version, he might consider selling one if enough people want it.
If not download and run the theme, then just copy the outputted HTML code it creates to use in your non-WP site. You will have to make a helluva lot of changes and strip things out though, as @mcjohnst said.
Going the “View Source -> Select All -> Paste” route is an option. An incredibly dirty option, but one I must shamefully admit I’ve done in the past. Once the bossman hits the printers up for a few hundred flyers with “Visit page.html!” on them, it’s a done deal at that point.
As @patrickzdb said, check with the theme’s support forum and see if an HTML version is available first.
If not, I’d fire up a local install using M/L/WAMP. I would also read the theme’s documentation and import any demo content or data into the theme so you have a good selection of page types available. Many people tend to forget that the demos they see on TF are with as many options and configurables crammed in there as possible.