The remote server returned an error: (502) Bad Gateway

try
{
  DOMFlightAvailabilityPortTypeClient obj = new DOMFlightAvailabilityPortTypeClient();
  string req = "<Request>"
  + "<Origin>BOM</Origin>"
  + "<Destination>DEL</Destination>"
  + "<DepartDate>2014-11-20</DepartDate>"
  + "<ReturnDate>2014-12-21</ReturnDate>"
  + "<AdultPax>1</AdultPax>"
  + "<ChildPax>0</ChildPax>"
  + "<InfantPax>0</InfantPax>"
  + "<Currency>INR</Currency>"
  + "<Clientid>12345</Clientid>"
  + "<Clientpassword>12345</Clientpassword>"
  + "<Clienttype>12345</Clienttype>"
  + "<Preferredclass>E</Preferredclass>"
  + "<mode>ONE</mode>"
  + "<PreferredAirline>AI,G8,IC,6E,9W,S2,IT,9H,I7,SG</PreferredAirline>"
  + "</Request>";

  string str = obj.getAvailability(req);
  XmlDocument doc = new XmlDocument();
  doc.LoadXml(str);
  DataSet ds = new DataSet();
  ds.ReadXml(new XmlTextReader(new StringReader(str)));
  GridView1.DataSource = ds.Tables[0];
  GridView1.DataBind();
}
catch (WebException ex)
{
  if (ex.Status == WebExceptionStatus.ProtocolError)
  {
     WebResponse resp = ex.Response;
     using (StreamReader sr = new StreamReader(resp.GetResponseStream()))
     {
         Response.Write(sr.ReadToEnd());
     }
  }
}