NEW YORK ( TheStreet) -- Shares of Southern Union ( SUG) soared late Thursday after receiving an all-cash buyout proposal from Williams ( WMB) that values the natural gas pipeline company at $4.9 billion, or $39 a share.

William's bid tops an existing agreement Southern Union has to be acquired by Energy Transfer Equity ( ETE)(ETE_) for $4.2 billion, or $33 a share.

Both the Williams' and Energy Transfer offers include the assumption of $3.7 billion of debt.

In a statement late Thursday, Southern Union said its board will "review the Williams' bid " in due course, consistent with fiduciary duties and in accordance with the terms and conditions of Southern Union's previously announced merger agreement with Energy Transfer."

Shares of Southern Union rose to $39.20, up 14.8%, in after-hours trading on Thursday.

-- Written by Joseph Woelfel

>To contact the writer of this article, click here: Joseph Woelfel

>To submit a news tip, send an email to: tips@thestreet.com.

If you liked this article you might like

Energy Transfer Equity, L.P. And Southern Union Announce Successful Completion Of Merger (Transcript)

Natural Gas Prices Poised to Go Higher

$100 Oil No Cause for Panic

Oil Prices Surge as Key U.S. Artery Opens

Oil Prices Poised for Jump Above $100