How to Render Partial View into a String

I have the following code:

public ActionResult SomeAction()
{
    return new JsonpResult
    {
        Data = new { Widget = "some partial html for the widget" }
    };
}

I'd like to modify it so that I could have

public ActionResult SomeAction()
{
    // will render HTML that I can pass to the JSONP result to return.
    var partial = RenderPartial(viewModel); 
    return new JsonpResult
    {
        Data = new { Widget = partial }
    };
}

is this possible? Could somebody explain how?

note, I edited the question before posting the solution.

This is a slightly modified version of an answer that works:

public static string RenderPartialToString(string controlName, object viewData)
{
    ViewPage viewPage = new ViewPage() { ViewContext = new ViewContext() };

    viewPage.ViewData = new ViewDataDictionary(viewData);
    viewPage.Controls.Add(viewPage.LoadControl(controlName));

    StringBuilder sb = new StringBuilder();
    using (StringWriter sw = new StringWriter(sb))
    {
        using (HtmlTextWriter tw = new HtmlTextWriter(sw))
        {
            viewPage.RenderControl(tw);
        }
    }

    return sb.ToString();
}

Usage:

string ret = RenderPartialToString("~/Views/MyController/MyPartial.ascx", model);