Disable New Line in Textarea when Pressed ENTER

try this

$("textarea").keydown(function(e){
// Enter was pressed without shift key
if (e.keyCode == 13 && !e.shiftKey)
{
    // prevent default behavior
    e.preventDefault();
}
});

update your fiddle to

$(".Post_Description_Text").keydown(function(e){
if (e.keyCode == 13 && !e.shiftKey)
{
  // prevent default behavior
  e.preventDefault();
  //alert("ok");
  return false;
  }
});