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: email@example.com.