CIblockSection::GetList:

 $arFilter = array('IBLOCK_ID' => $IBL_ID);

  $rsSect = CIBlockSection::GetList(array('left_margin' => 'asc'),$arFilter);

  if ($arSect = $rsSect->GetNext())

      $arLoadProductArray['IBLOCK_SECTION'] = $arSect['ID'];